1. Field of the Invention
The subject of the present invention is a process for the manufacture of implants or prosthetic elements with an even madreporic surface (i.e., a surface roughened by small balls). This surface has part spherical protuberances formed during the casting process.
2. Description of the Prior Art
Surgical prostheses, particularly those for joint articulation, such as those of the knee or hip, are usually implanted on or in broken bones, the parts thereof which are in a deteriorated condition having been removed with a surgical saw.
It is essential, with this type of implanted prostheses, that the prosthetic parts in contact with the resected parts of the bone which is in good condition, can be integrated into the bone in the process of reconstitution such as by the ingrowth of bone tissue.
To achieve this end, it is already known how to resort to implants made of material compatible with the bones and how to provide that the surfaces of these implants facilitate the implantation of the bony tissue undergoing regeneration.
It has thus been proposed, for example in the applicant's U.S. Pat. No. 4,355,428, the teachings of which are incorporated herein by reference, that the surfaces of these implants be rendered madreporic, i.e., that they be constituted by a multitude of small balls, all of which form an integral part with the generally metallic material constituting the prosthetic implant itself.
In the process of the '428 patent, prior to casting using the lost wax technique, meltable balls, which generally have a certain range of diameter, are sprinkled on the surface of a wax model of the implant precoated with glue.
During such sprinkling, the greater possible heterogeneity of the surface is favored by using balls of the same diameter or different diameters and distributing them, generally by hand, in an uneven manner so as to increase as much as possible the intervals, crevices or interstices between neighboring balls, in order to promote the ingrowth and development of the bone fibrils being reformed after implantation of the prosthesis.
The wax prosthesis model thus obtained is covered by ceramic which makes possible the production of a female mold of refractory material into which, after melting of the wax and balls--hence the name "lost wax process"--the metal or alloy constituting the prosthesis can be poured and the corresponding prosthesis with the sought-after surface appearance obtained.
It was found, however, that such a production process for prostheses, already burdensome because it requires the preparation of a wax model for each prosthesis to be made, was very much dependent on the operator doing the manual sprinkling, gluing, and fixation of the balls on the wax model. This results in prostheses whose surface appearance exhibits the desired irregularities, but whose order of magnitude varies considerably from one prosthesis to the next and did not reliably allow for their identical reproduction.